Arthritis of the hip primarily affects the articulation between the femoral head and the acetabulum. If surgery is required, the primary objective is to replace the articulating surfaces. To achieve this it is desirable to obtain a homogeneous transfer of forces to the proximal femur. This is best provided by retention of the femoral neck. Early attempts to achieve a conservation of the femoral neck and more physiological loading were betrayed by poor materials, inadequate fixation and failure of the articulation. Despite the high level of survivorship of cemented and uncemented stemmed femoral components, there is a need for a joint prosthesis that does not invade the femoral canal. The above mentioned prosthesis would be useful in treating younger and more active patients, in whom the use or such a design would usefully prolong the time where a conventional total joint replacement design is necessary.
While the more frequent use of hard-hard articulations and the highly cross-linked polyethylenes in total hip replacement are anticipated to lead to a reduction of osteolysis, in addition to wear, stress shielding is expected to become a more targeted cause of bone reduction. The reduction of stress and strain in the proximal femur following total hip replacement is hypothesized to be one reason of proximal bone loss, which may lead to a reduction of implant support, progressive implant subsidence and periprosthetic bone fracture. Even though it is not clear if the resorption of the proximal femoral bone stock is directly related to the survival of implants, an excessive bone loss around a primary prosthesis can reduce the longevity of a revision prosthesis by compromising the bone stock available at the revision procedure. Thus, conservation of bone stock is a vitally important principle, especially in young patients where the chances of revisions during the patients' lifetime are high. Particularly in uncemented total hip replacement, which is often favored in young patients, the stem geometry is believed to play an important role in the load transfer to the femur and, consequently, in femoral remodeling. As canine studies, periprosthetic bone mineral density measurements and clinical observations have indicated, implantation of different femoral stems lead to a bone reaction specific for the geometry, surface finish and stiffness of the implants used.
The main problem with the replacement procedure is the survival rate and the revision options. At a revision there is a considerable amount of bone of the upper femur destroyed in the loosening process, and during the removal procedure of the existing femoral component. The subsequent revision implant is necessarily larger and longer in order to gain sufficient fixation. The survivorship of such devices is usually less than that of the primary procedure. Moreover, should that device fail, the prognosis is very poor indeed. Hence, there is a strong rationale for use of a “conservative” device at the primary stage, which involves interfacing with far less of the femur than does a conventional total hip. The goals of such a conservative device are that it will be easy to insert and will have a survivorship similar to that of a conventional total hip. Even if the survivorship was slightly less, there is still a justification for its use. If a conservative hip is suitably designed and if it were to fail by loosening or other reason, then its removal would involve little destruction of the femoral bone. The revision procedure would then be equivalent to the use of a primary total hip. Thereby, the patient would have gained a substantial time period, say ten years or more. The high probability of revision in these younger more active patients has been one of the main factors driving the quest for more bone sparing conservative options at a total hip replacement. The present invention therefore seeks to address this issue.